CANCUN CLIMATE SUMMIT

Data for 60,000 ships, including many of the big, long-distance carriers, has been put in to the website using data from international registers and methods developed by the International Maritime Organisation (IMO).

The project's initiators hope that big corporations in particular will selectively use low-carbon carriers, encouraging all operators to improve their operations and reducing the industry's overall carbon footprint.

"We're hoping that companies like Nike or Walmart will go for it for two reasons," said Peter Boyd of the Carbon War Room.

"Firstly, they're concerned about greening their brands, but also about securing their supply chains."

But, he said, he was also intrigued by the idea that governments could set differential landing charges for ships depending on their emissions.

Papua New Guinea's delegate to the UN climate convention meeting, Kevin Conrad, told BBC News his government was considering the idea as part of a bigger package of measures designed to cut carbon through engagement with the private sector.

"Our duty is to find those that are leading the charge in the private sector, and work with them to achieve our climate goals," he said.

The ships would be rated on an A-G scale according to their efficiency.

The scheme's labels look very similar to the ratings given to consumer electrical goods such as refrigerators in the EU, which have helped drive up standards.

The Carbon War Room - a non-profit organisation aiming to "harness the power of entrepreneurs" to curb climate change - is hoping that ship owners will voluntarily choose to lodge their emissions data on the website shippingeffiency.org in order to boost their profile.

They calculate that global shipping emissions could be cut by about 30% just through increasing efficiency, although much greater gains could materialise in future as designers pursue new - or revisit old - concepts such as sails, kites and solar power.

Ships could be charged different fees to dock depending on how much carbon they emit, according to ideas being discussed on the sidelines of the UN climate summit.